It's not just your 3DS. Once in a while, I am next to someone who has Streetpass activated and that Mii simply won't show up. I have tried changing settings, switching Miis, etc., but the Mii eventually shows up if I keep the 3DS on long enough.
As for your earlier post, that happens to me all the time (getting the tag for a game but not for the Mii Plaza). A few weeks ago, I asked if anyone else had this problem, and noone did. There isn't a fix, and my experience with Streetpass shows me that it isn't a perfect program.
Here is one thing that may help you. I often find that, after a prolonged period of playing a game online, quitting to the Home menu and immediately putting the 3DS into sleep mode to get Streetpasses does not work. I have tried turning off the system, letting it rest for a minute, and then turning it back on again to get more tags. This has only given me mixed results, but it does sometimes help me detect nearby 3DS's that I previously couldn't connect to.
